Infections by the Ebola virus can cause severe viral hemorrhagic fever resulting in the suppression of host innate and adaptive immune systems. The Ebola virus inhibits host interferon (IFN) activities in order to replicate, thus resulting in serious disease with fatality rates as high as 90%. The ability of Ebola to rapidly generate viral components while inhibiting host antiviral repsonses depends on a few multifunctional viral proteins, including viral protein 35 (VP35). VP35 can inhibit IFN regulatory factor-3 (IRF-3) activity through its ability to bind double stranded RNA (dsRNA). However, the mechanisms and the molecular details of how VP35 binds dsRNA and promotes antiviral antagonism are not known, partly due to the lack of structural knowledge on viral components. The objective of this proposal is to identify the biochemical and structural determinants that regulate VP35/dsRNA interactions, which facilitate VP35 mediated host immune evasion. We recently solved the structure of the VP35 IFN inhibitory domain (IID), which revealed a critical extended patch of basic residues centered around R312 that are required for dsRNA binding. Following these developments, I will first solve the structures of two mutant VP35 IID protein in order to identify key structural elements that required for RNA-dependent and RNA-independent functions of VP35. Together, these studies are designed to provide a comprehensive understanding of the biochemical and structural basis for antiviral antagonism of VP35 through interactions with dsRNA. Moreover, these studies will provide quantitative structural and biochemical data, which will be used to develop allosteric regulators of the VP35 function with potential applications as therapeutic and cell biological reagents. The potential use of the Ebola virus as a bioterrorist agent, coupled with the lack of current approved treatments for infections, makes the Ebola virus a significant threat to global human health.